Load cells or strain gage transducers are example force measurement devices commonly used in a variety of industries. A shear-web load cell or strain gage design or configuration is typically used in large force level load cells. An important component in the load cell, or strain gage transducer, is generally the spring element. Broadly stated, the function of the spring element is to serve as the reaction for the applied load. Implicit in this definition is the assumption that the strain level in the gaged area of the spring element responds in a linear-elastic manner to the applied load. In other words, the ideal transducer would be characterized by an unvarying, proportional relationship between the strain and the load. See Strain Gage Based Transducers, Chapter 2, “Load Cells” by Measurements Group, Inc. (1998).
More specifically, and as illustrated in FIG. 1A, a shear-web based spring element of the load cell or strain gage may take the form of a cantilever beam, which has been designed with a generous cross-section to minimize deflection. At section A-A of the beam, a recess has been machined in the side, leaving a relatively thin web in the center. Most of the shear force imposed by the load is carried by the web, while the bending moment is resisted by the flanges.
Referring now to FIG. 1B, a cross-section A-A is depicted, along with shear and bending stress distributions in this cross-section. At the neutral axis, where the bending stress is zero, the state of stress on the web is one of pure shear, acting in the vertical and horizontal directions.
Shear-web spring elements are not limited to cantilever-beam configurations. For example, FIG. 1C illustrates a simply supported beam because of the flexures or cut-away sections at both ends. In other words, FIG. 1C depicts dual shear webs in a beam configuration that are designed to cancel the effects of the off-axis and side loads.
Several design considerations are generally applicable to all load cell spring elements including: (1) appropriate strain level in the gage area at rated load; (2) uniform strain distribution in the strain gage area; (3) lower strain levels throughout the remainder of the spring element; (4) monolithic (one-piece) construction; (5) design for ease of machining and gage installation; (6) spring element deflection; and (7) thermal considerations. See Strain Gage Based Transducers, Chapter 2, “Load Cells” by Measurements Group, Inc. (1998).
In nuclear power plants and other valve installations, it is desirable to have an independent verification of the force or load applied by an actuator on a valve stem to allow for more precise diagnostics. Several solutions are available. One is to install a known strain gage or load cell directly on the valve stem. This procedure, however, is time consuming and requires special chemicals and techniques to achieve a consistent, accurate gage installation. The measured force values must be inferred from the properties of the material of the valve stem, and no provision is made for corrections due to ambient temperature or creep.
A second method involves buying a valve stem that already has a strain gage installed inside. This method allows for calibration and many of the compensation features that are common in load cell configurations, but is not viable for stems that are welded to the valve plug. In addition, the cost for custom stem configurations can be high.